[Openicc] Ghostscript CMS [was: PDF frustration]

[Michael Vrhel]

> -----Original Message-----
> From: openicc-bounces at lists.freedesktop.org [mailto:openicc-
> bounces at lists.freedesktop.org] On Behalf Of Gerhard Fuernkranz
> Sent: Monday, December 01, 2008 1:01 PM
> To: 'OpenICC Liste'
> Cc: 'Ralph Giles'
> Subject: Re: [Openicc] Ghostscript CMS [was: PDF frustration]
> 
> Kai-Uwe Behrmann wrote:
> > Am 01.12.08, 11:47 +0100 schrieb Leonard Rosenthol:
> >> On Dec 1, 2008, at 9:31 AM, Kai-Uwe Behrmann wrote:
> >>>> The idea of the rendering intent is just to communicate to the CMS
> what is
> >>>> requested for those colors in the document.  In PDF, you can set a
> >>>> rendering
> >>>> intent.  The CMS will need to decide what it wants to do with that
> >>>> information.  It may decide for perceptual intent for example that it
> will
> >>>> do black point compensation.
> >>> Bpc works only with relative colorimetric.
> >> 	Are you sure about that?  In all of the discussions around BPC, I
> >> have never heard anyone say this before.
> >
> > http://www.adobe.com/devnet/photoshop/sdk/AdobeBPC.pdf section 6.2 says
> > that relative colorimetric is the primary target, as it has no mapping
> > from source black to destiniation black defined. Perceptual and
> saturation
> > should have this per definition. There is a advantage mentioned for the
> > case of malformed profiles.
> > I agree that the relative colorimetric with BPC combination is not the
> > only allowed per this document, just from a practical point of view the
> > others combinations should play no role.
> 
> Kai-Uwe,
> 
> IMO, for perceptual/saturation intent a BP scaling may make sense in
> conjunction with certain V2 profiles. The perceptual/saturation intent
> of some (most?) V2 profiles seems to map "device black" to L*a*b* =
> 0,0,0 in the PCS, while some other V2 profiles seem to map "device
> black" to L* > 0 [e.g. to the actual device BP, adapted to D50]. I'm not
> sure, what's right or wrong, IMO in V2 it is not clearly defined.
> Anyway, profiles which assume a different perceptual BP in the PCS do
> not work well together w/o BPC when they are combined with
> perceptual/saturation intent. When combining V4 profiles, this should
> not be an issue any longer, since V4 has a well-defined perceptual BP in
> the PCS, and the perceptual/saturation intent tables of V4 profiles are
> expected to map device black to this well-defined perceptual BP in the
> PCS.
> 
> Best Regards,
> Gerhard
> 
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I will need to review what is going on wrt to BPC.  You are correct Gerhard,
that many if not most V2 profiles did a BPC mapping in the profile itself.
At lease that was what the software (Color Savvy) that I worked on years ago
to create V2 profiles.  If there is an option in a PDL to specify that BPC
is needed we will certainly pass that information along.  If there is not,
then we need to think a bit about how to best handle that and any other
special settings that the CMM may have.  This is obviously dependent upon
the CMM and may find its way into the ghostscript API where some structure
of options is setup for the CMM.